Distinct reaction pathways followed upon reduction of oxy-heme oxygenase and oxy-myoglobin as characterized by mössbauer spectroscopy

Activation of O₂ by heme-containing monooxygenases generally commences with the common initial steps of reduction to the ferrous heme and binding of O₂ followed by a one-electron reduction of the O ₂-bound heme. Subsequent steps that generate reactive oxygen intermediates diverge and reflect the effects of protein control on the reaction pathway. In this study, Mössbauer and EPR spectroscopies were used to characterize the electronic states and reaction pathways of reactive oxygen intermediates generated by 77 K radiolytic cryoreduction and subsequent annealing of oxy-heme oxygenase (HO) and oxy-myoglobin (Mb). The results confirm that one-electron reduction of (Fe^II-O₂)HO is accompanied by protonation of the bound O₂ to generate a low-spin (Fe ^III-O₂H^-)HO that undergoes self-hydroxylation to form the α-mesohydroxyhemin-HO product. In contrast, one-electron reduction of (Fe^II-O₂)Mb yields a low-spin (Fe ^III-O₂^2-)Mb, Protonation of this intermediate generates (Fe^III-O₂H^-)Mb, which then decays to a ferryl complex, (Fe^IV=O^2-)Mb, that exhibits magnetic properties characteristic of the compound II species generated in the reactions of peroxide with heme peroxidases and with Mb. Generation of reactive high-valent states with ferryl species via hydroperoxo intermediates is believed to be the key oxygen-activation steps involved in the catalytic cycles of P450-type monooxygenases. The Mössbauer data presented here provide direct spectroscopic evidence supporting the idea that ferric-hydroperoxo hemes are indeed the precursors of the reactive ferryl intermediates. The fact that a ferryl intermediate does not accumulate in HO underscores the determining role played by protein structure in controlling the reactivity of reaction intermediates.